About 5 billion years ago, a great
pancake-shaped cloud of gas and dust circled around the newborn Sun. Slowly, the
planets as we know them today grew from the material of this cloud.

From the cold parts of the cloud, far away from the Sun, came the icy giants.
These are big planets that took advantage of the low temperatures to attract –
and hold on to – light gases such as hydrogen.

From the warmer parts, close to the Sun, came the rocky worlds. These are planets
whose gravity pulls were too weak, and temperatures too high, to let them build
light, gassy bodies of great size.

All the planets from Mercury to Neptune fit nicely into this picture of how the
Solar System formed. but there's one other planet – Pluto – that doesn't.

The ancient Greek god Pluto, king of the underworld, lived in a place that was
dark and unfriendly. Because the planet Pluto is in such a dark, faraway place
in the Solar System, it was named after him.

At a distance of 3.6 billion miles (5.9 billion kilometers). Pluto is usually
the planet farthest from the Sun. But its path around the Sun is unusual. It is
much more oval, or stretched out, than the near-circular paths of the other planets.
In fact, at times during its very long year – lasting 248 Earth-years –
Pluto is closer to the Sun than Neptune.

Another strange thing about Pluto is its size. Far from being a giant like Neptune
and Uranus, its closest neighbors, Pluto is actually the smallest planet in the
Solar System. At less than 2,000 miles (3,200 kilometers) in diameter, it's even
smaller than the Moon.

Pluto probably has a tiny, rocky core that is surrounded by layers of frozen gases
such as hydrogen and methane. It's the coldest planet of all, with a surface temperature
close to -400°F (-240°C).

Scientists now think that Pluto came from a big swarm of icy objects, called the
Kuiper Belt, that lies in the far reaches of the Solar System. Because of its
small size, Pluto has been renamed a "dwarf planet." But although it is very small,
Pluto does have three moons of its own. The first of these to be discovered, and
the largest, is called Charon. That's the name of the boat man who ferried people
across the river Styx on the way to Pluto's underworld. Like the boat man, Charon
moves close by a dim, faraway world.

What a strange sight it would be to look at the sky from the frozen wasteland
of Pluto! Less than 12,000 miles (19,000 kilometers) away, Charon would appear
17 times bigger than the full moon from Earth. More than 3½ billion miles
away, the Sun would be just a bright point of light – the only friendly
reminder of home.

The New Frontier

We are lucky to live at a time, and in a part of space, that is so filled with
interest. Until quite recently, we knew very little of the strange worlds of the
Solar System around us. We could only watch them dimly through our telescopes.
But now we have begun to explore many of them with spacecraft. We have seen their
amazing surfaces close up, thanks to the cameras and other instruments of our
robot explorers. Perhaps within our lifetimes, we will journey to the planets
ourselves and explore firsthand this exciting new frontier.